1. Field of the Invention
The present invention generally relates to the calendering of a web of paper, paperboard or the like. More specifically, the invention relates to a calender belt, of the kind used on a calendering system at the downstream end of a paper machine or on an off-machine calender, which passes, together with the web, through a calender nip to provide a desired finish to the web.
2. Description of the Prior Art
Paper or paperboard is calendered during manufacture in order to be provided with increased surface smoothness and gloss. Calendering is required to provide many printing papers with a desired printing quality, and may be carried out on both coated and uncoated paper or paperboard.
Calendering may be performed on-line on a papermaking or board machine immediately after the dryer section thereof. In on-line calendering, a machine calender comprising at least one calender nip formed between two hard rolls may be used. Machine calendering is also known as hard calendering, because both press rolls are hard.
Calendering can also be performed off-line, substantially separate from the papermaking or board machine. In such case, use is traditionally made of a so-called supercalender, which comprises a relatively large number of rolls arranged in a vertical stack. Usually, every other roll in a supercalender is hard, and those between the hard rolls are of a softer material, so that the side of the web contacting the hard rolls receives increased gloss. A more uniform treatment of the web can be achieved if the relative positions of the hard and soft rolls are exchanged at the center of the supercalender, so that the side of the web originally contacting the soft rolls may contact the hard rolls.
Calenders with elastic rolls, or soft calenders, have also been developed for on-line calendering. A soft calender, also known as a compliant calender, can be disposed on-line after the papermaking or board machine or a coating unit, and normally has a relatively small number or rolls. In compliant calendering, each nip is formed between a heated steel roll and an associated elastic roll, such as a polymer-coated roll. Heat, which makes the web soften in the nip, is supplied to make the paper web as smooth and glossy as it would become if a supercalender were used. The elasticity of the elastic roll in a soft calender permits the press nip to become somewhat extended. In turn, this extension leads to a flattening of the pressure pulse relative to that of a machine calender, so that the compression on the paper web can advantageously be limited as compared with that in a machine calender.
The results obtained in machine (hard) calendering, using two hard rolls, and compliant (soft) calendering, using one hard, heated roll and one elastic roll, are different from one another. A machine calender with hard rolls calenders to a constant web thickness. The undesired consequence of constant web thickness is a non-uniform density in the calendered web because the high, localized pressure pulse imparted in the press nip gives a comparatively stronger compression to the thicker portions of the web. A compliant calender, however, calenders to a more constant web density. The consequence, however, is a web which is not of uniform thickness, and can have poorer gloss and smoothness.
In either case, the calendered paper sheet is non-uniform in some respect. Accordingly, it may be necessary, depending on the contemplated use of the calendered paper or paperboard, to make a trade-off between non-uniform thickness and non-uniform density, as each has its own effect on the quality of the images printed on the paper or paperboard.
Compliant (soft) calenders which incorporate an endless calender belt, rather than a polymer-coated roll, have been developed. The calender belt passes in an endless path around a roll which forms a pressure nip with a hard roll. In operation, the paper or paperboard web is located in the nip between the elastic, endless belt and the hard roll. A benefit of this design can be that the calender belt, which is heated in the nip by heat from the heated, hard roll, can be cooled during its return in the closed loop.
Calenders of substantially the same design as long nip presses for the press sections of paper machines have also been used in compliant calendering. Compliant calenders of this type have an extended nip formed between a rotating and often heated hard roll and a matching, substantially stationary, concave support element or press shoe. The paper or paperboard web passes through the nip along and in contact with a support medium in the form of an endless calender belt, which in the nip is located between the web and the support element or shoe. The calender belt passes in an endless path around the support element or shoe and, as in this kind of press in a press section, must be impermeable on the shoe side also.
Endless calender belts for soft calendering are traditionally made of a woven base structure impregnated to a desired thickness, either on one or both sides, with a suitable impregnating substance, generally polyurethane. It will be appreciated, in view of the preceding discussion on the effects calendering has upon a paper web, that the properties of the calender belt must be uniform in order not to introduce or to worsen non-uniformities in the calendered paper web. Since the paper or paperboard web is in direct contact with the calender belt, it must have a very smooth surface to impart the desired finish characteristics to the paper or paperboard web. In particular, the elastic modulus and the elastic deformation/recovery in the Z-direction, that is, the direction perpendicular to the plane of the calender belt, must be proper and uniform to ensure that all parts of the paper web experience the same pressure pulse in the pressure nip.
Heretofore, one of the shortcomings of the calender belts currently in use has been a non-uniform structure. The principal reason for the difficulty in providing a uniform structure, it has been discovered, is that the polymeric impregnating substance does not uniformly impregnate the base of the calender belt. As a consequence, the response of the calender belt to compression varies across the surface of the calender belt. In turn, these variations cause the shape of the pressure pulse at points across the pressure nip to vary periodically, and, as a consequence, cause the thickness, density, smoothness and gloss of the calendered paper web to be non-uniform.
A second shortcoming of the calender belts currently in use is a lack of structural integrity. In any coated fabric having a resin coating mechanically bonded to the yarns of a woven base structure, delamination of the resin coating can occur. If the resin coating is applied in more than one layer, such as in a multiple thin pass (MTP) process, there is also the possibility of interlayer delamination caused by shear stresses imposed on the calender belt as it passes through the nip of the calender, or at specific locations across the nip known as stress concentrators. These locations can be at roll edges; a roll surface where the surface "dubbing" is slightly incorrect; or at the shoe edges, where the calender belt may take a complex bend.
Another shortcoming of the calender belts currently in use is stress cracking and crack propagation within the resin coating. This consequence of fatigue in the resin coating usually begins at the location of a stress concentrator, or may just be due to a combination of shear and compressive fatigue. Hysteresis can also be a factor. Once cracks begin to appear, they can propagate across the surface and deepen into the resin coating, eventually allowing pieces of the resin coating to wear away quickly and nonuniformly, and necessitating the removal and replacement of the calender belt.
Still another shortcoming of the calender belts currently in use is that there is an upper limit to the thickness of the resin coating that can be applied. An overly thick layer is susceptible to failure due to shear forces and hysteresis. Yet a thick layer is often needed to meet the requirements of a particular calender nip and/or the paper grade properties being developed.
The present invention is an improved calender belt compared to the calender belts of the prior art, and represents a solution to the above-noted deficiencies of those calender belts.